SPORTS MEDICINE

Prevention of anterior cruciate ligament injuries in sports—PartI: Systematic review of risk factors in male athletes

Eduard Alentorn-Geli • Jurdan Mendiguchıa •

Kristian Samuelsson • Volker Musahl •

Jon Karlsson • Ramon Cugat • Gregory D. Myer

Received: 6 August 2013 / Accepted: 14 October 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract

Purpose The purpose of this study was to report a com-

prehensive literature review on the risk factors for anterior

cruciate ligament (ACL) injuries in male athletes.

Methods All abstracts were read and articles of potential

interest were reviewed in detail to determine on inclusion

status for systematic review. Information regarding risk

factors for ACL injuries in male athletes was extracted

from all included studies in systematic fashion and classi-

fied as environmental, anatomical, hormonal, neuromus-

cular, or biomechanical. Data extraction involved general

characteristics of the included studies (type of study,

characteristics of the sample, type of sport), methodologi-

cal aspects (for quality assessment), and the principal

results for each type of risk factor.

Results The principal findings of this systematic review

related to the risk factors for ACL injury in male athletes

are: (1) most of the evidence is related to environmental

and anatomical risk factors; (2) dry weather conditions may

increase the risk of non-contact ACL injuries in male

athletes; (3) artificial turf may increase the risk of non-

contact ACL injuries in male athletes; (4) higher posterior

tibial slope of the lateral tibial plateau may increase the risk

of non-contact ACL injuries in male athletes.

Conclusion Anterior cruciate ligament injury in male

athletes likely has a multi-factorial aetiology. There is a

lack of evidence regarding neuromuscular and biome-

chanical risk factors for ACL injury in male athletes.

Future research in male populations is warranted to provide

adequate prevention strategies aimed to decrease the risk of

this serious injury in these populations.

Level of evidence Systematic review on level I–IV stud-

ies, Level IV.

Keywords Prevention � ACL injury � Risk factors �Male athletes

E. Alentorn-Geli (&)

Department of Orthopaedic Surgery, Hospital del Mar—Parc de

Salut Mar, Universitat Autonoma de Barcelona and Universitat

Pompeu Fabra, Passeig Marıtim 25-29, 08003 Barcelona, Spain

e-mail: ealentorngeli@gmail.com

J. Mendiguchıa

Department of Physical Therapy, Zentrum Rehab and

Performance Center, Baranain, Navarre, Spain

K. Samuelsson � J. Karlsson

Department of Orthopaedics, Sahlgrenska University Hospital,

Molndal, Sweden

V. Musahl

Department of Orthopaedic Surgery, University of Pittsburgh

Medical Center, Pittsburgh, PA, USA

R. Cugat

Garcıa-Cugat Foundation, Barcelona, Spain

R. Cugat

Mutualidad de Futbolistas, Federacion Espanola de Futbol—

Delegacion Cataluna, Barcelona, Spain

G. D. Myer

Division of Sports Medicine, Cincinnati Children’s Hospital

Medical Center, Cincinnati, OH, USA

G. D. Myer

Departments of Pediatrics and Orthopaedic Surgery, University

of Cincinnati, Cincinnati, OH, USA

G. D. Myer

The Micheli Center for Sports Injury Prevention,

Boston, MA, USA

123

Knee Surg Sports Traumatol Arthrosc

DOI 10.1007/s00167-013-2725-3

Introduction

Anterior cruciate ligament (ACL) injuries are a common

and debilitating injury in competitive sports [10, 37, 67].

The costs of this injury are very high from both the

patient’s health and economic standpoints [33, 35, 36, 49].

The high incidence of ACL injury along with the poten-

tially devastating long-term consequences on the knee has

led to great research efforts focused on prevention and

management of injuries. Many studies have been con-

ducted to better understand the inciting mechanisms of

ACL injury and the associated risk factors that contribute

to increased risk for injury [3, 17, 38, 42]. While sub-

stantive research efforts have been made to improve

diagnosis and treatment strategies [5, 15, 19, 24, 45, 53, 55,

56, 72, 77], the long-term consequence of ACL injuries is

likely osteoarthritis, even in cases undergoing surgical

treatment [49]. Therefore, the most effective strategy to

avoid joint damage is prevention.

There has been a growing awareness on the relevance

of prevention of ACL injuries in athletes from health care

providers who are faced to manage the injuries. Women

have a higher risk of non-contact ACL injuries compared

to their male counterparts [6, 10]; therefore, research on

mechanisms of injury, risk factors and prevention strate-

gies of these injuries has predominately focused on the

female athlete [3, 4, 17, 18, 40, 42]. Most of the inves-

tigations only included females or compared females and

males [3, 4, 17, 18, 40, 42, 56]. However, the overall

incidence of ACL injuries is high in males as well [54],

especially considering that males may be more exposed to

high-risk sports [33]. Unfortunately, the available infor-

mation on risk factors for non-contact ACL injuries spe-

cifically in male has not been well documented to date.

Thus, the exact information regarding the factors that

place a male athlete at greater risk of ACL injury is not

well understood and data from female athletes are often

extrapolated to their male counterparts, which likely leads

to sub-optimal intervention and management strategies for

the male athlete. Therefore, the purpose of this systematic

review was to assimilate and synthesize the existing

evidence on the risk factors for ACL injuries in male

athletes.

Material and methods

The methodology of this study was reported following the

Preferred Reporting Items for Systematic Reviews and

Meta-Analyses (PRISMA) Statement for systematic

reviews [52]. This is an evidence-based minimum set of

items that serve as a guide for authors when reporting

systematic reviews or meta-analyses, and include: title,

abstract, introduction (rationale and objectives), methods

(protocol and registration, eligibility criteria, information

Table 1 Search strategy and keywords employed for the systematic

literature search

Database Search Keywords/query

Pubmed 1 (anterior cruciate ligament OR ACL) AND

(injuries OR injury OR tear OR tears OR

tearing OR rupture OR ruptures)

2 Sports[mesh] OR sports[tw] OR sport[tw] OR

athletes[mesh] OR athletes[tw] OR

athlete[tw]

3 #1 AND #2

4 ((animals[mh]) NOT (animals[mh] AND

humans[mh]))

5 #3 NOT #4

6 (Editorial[ptyp] OR Letter[ptyp] OR

Comment[ptyp])

7 #5 NOT #6

8 #5 NOT #6 Filters: English

EMBASE 1 Exp anterior cruciate ligament/

2 (anterior cruciate ligament OR ACL). ti, ab,

kw

3 #1 OR #2

4 Exp injury/

5 (injuries OR injury OR tear OR tears OR

tearing OR rupture OR ruptures). ti,ab,kw

6 #4 OR #5

7 Exp sport/

8 Exp athlete/

9 (sports OR sport OR athletes OR athlete). ti,

ab, kw

10 #7 OR #8 OR #9

11 #3 AND #6 AND #10

12 Limit 11 to (EMBASE AND English AND

(article OR conference paper OR ‘‘review’’))

Cochrane

library

1 MeSH descriptor: [Anterior Cruciate

Ligament] explode all trees

2 Anterior cruciate ligament OR ACL: ti, ab, kw

(word variations have been searched)

3 #1 OR #2

4 MeSH descriptor: [Wounds and Injuries]

explode all trees

5 Injuries OR injury OR tear OR tears OR

tearing OR rupture OR ruptures: ti, ab, kw

(word variations have been searched)

6 #4 OR #5

7 MeSH descriptor: [Sports] explode all trees

8 MeSH descriptor: [Athletes] explode all trees

9 Sports OR sport OR athletes OR athlete: ti, ab,

kw (word variations have been searched)

10 #7 OR #8 OR #9

11 #3 AND #6 AND #10

Knee Surg Sports Traumatol Arthrosc

123

sources, search, study selection, data collection process,

data items, risk of bias in individual studies, summary

measures, synthesis of results, risk of bias across studies,

and additional analyses), results (study selection, study

characteristics, risk of bias within studies, results of indi-

vidual studies, synthesis of results, risk of bias across

studies, and additional analysis), discussion (summary of

evidence, limitations, and conclusions), and funding.

Eligibility criteria

All prospective, cross-sectional or retrospective prognos-

tic human studies investigating risk factors for ACL

injury were evaluated for eligibility. Studies were inclu-

ded if they were: level of evidence between I and IV,

written in English language, and contained results speci-

fied for males. Studies only reporting issues other than

risk factors or studies investigating only female sample

were excluded. Studies not including ACL-injured sub-

jects or ACL injury as an outcome measure were exclu-

ded. Therapeutic and diagnostic studies were excluded.

Review articles, systematic reviews, and meta-analyses

were not included, but reference lists were examined to

ensure completeness of relevant studies. To avoid selec-

tion bias, studies comparing females and males were

reviewed in detail to assess for intra-group comparison in

males.

Information sources and search

Electronic search

A systematic electronic literature search was conducted

using the PubMed (MEDLINE) database and The Coch-

rane Library up to September 2012 (no start date), EM-

BASE database from 1980 to September 2012. Two expert

librarians in electronic search methods performed the lit-

erature search. The search strategy and keywords employed

in this study are summarized in Table 1.

Other search methods

The reference lists of all included articles were reviewed to

search for potential studies not previously identified.

Data collection and analysis

Study selection

All abstracts were read and articles of potential interest were

reviewed in detail (full text) by 3 authors (EAG, JM, and

GDM) to decide on inclusion or exclusion from this sys-

tematic review. In cases of disagreement, all three authors

reviewed and discussed the study and a final decision in

consensus. In all cases where the information regarding the

sex of the subject was not provided, a contact with the cor-

responding author was established to determine the sex of

the investigation subjects.

Data collection process

Information regarding risk factors for ACL injuries in

male athletes was extracted by the first author from all

included studies in systematic fashion following the

classification of risk factors suggested in the Hunt Valley

meeting [38]. Thus, risk factors were classified as envi-

ronmental, anatomical, hormonal, neuromuscular, and

biomechanical. Data extraction involved general charac-

teristics of the included studies (type of study, charac-

teristics of the sample, type of sport), methodological

aspects (for quality assessment), and the principal results

for each type of risk factor. One author (EAG) performed

all data extraction, which was then verified by two

authors (JM and GDM).

Assessment of the risk of bias

The methodological quality of all included studies was

evaluated with the assessment of the risk of bias in several

areas of a research project. This assessment was performed

by answering yes, no, unknown, or not applicable to the

following information from each study: prospective, con-

cealed allocation (yes = assignment was made by an

independent person who had no information about study

participants), similarities at baseline between groups

(yes = study groups were similar in demographic charac-

teristics), blinding of participants (yes = study population

blinding was clearly described and acceptable), blinding of

data collectors (yes = data collectors were blinded

regarding group assignments), blinding of outcome asses-

sors (yes = outcome assessors who evaluated the partici-

pants were blinded regarding group assignments), previous

knee injuries excluded, results specified for non-contact

injuries, influence of other risk factors controlled, accept-

able compliance (yes = compliance was regularly checked

or otherwise strictly supervised by someone other than

study participants, and it was more than 70 % in every

study group), dropout reasons reported, acceptable dropout

rate (yes = dropout rate was \30 %), duration of inter-

vention comparable between groups, and intention-to-treat

analysis (yes = all subjects assigned to a group at the

beginning of the study were included in the analysis). A

final quality score was given for each study, where ‘yes’

was 1 point and the other response was 0 points. The

assessment of the risk of bias in included studies was based

on the article by Aaltonen et al. [1].

Knee Surg Sports Traumatol Arthrosc

123

Results

Study selection

The literature search elicited a total of 3,322 references,

from which 1,495 were duplicates and another 1,785 were

excluded (Fig. 1). A total of 42 studies were reviewed in

full text, and 24 were included. In addition, ten articles

were added after reviewing the reference lists of included

studies. Therefore, 34 articles met the final inclusion cri-

teria for the current systematic review.

Characteristics of the studies

Of the 34 studies, 12 were prospective and 22 non-

prospective studies. Table 2 summarizes the assessment

of the risk of bias for the included studies. The

approximate number of participants involved in the 34

studies was 18,913 (some studies did not specify the

total number of participants included), although it is

likely that some subjects were employed for more than

one study published by the same group of authors.

Table 3 summarized all the evidence extracted from the

34 included studies.

External risk factors

Environmental risk factors

The evidence found for environmental risk factors for ACL

injuries in male athletes was primarily related to weather

conditions, playing surface and shoe characteristics. Envi-

ronmental are the risk factors in which this systematic

review found most of the references assessing risk factors

for ACL injuries in males. A total of 17 references were

found for environmental risk factors: 8 for weather con-

ditions, 12 for playing surface, and 1 for footwear (some

studies were classified in more than one group).

Weather conditions There were 3 studies investigating

weather conditions in American football, 4 in Australian

football, and 1 in soccer. The approximate number of

subjects involved in the study of weather conditions was

7,624.

Regarding American football, Scranton et al. [70]

investigated 22 teams of the National Football League

during 5 seasons and found an increased incidence of non-

contact ACL injuries in dry compared to wet conditions.

The National Football League was also analysed by

Bradley et al. [11] in a descriptive epidemiology study

(seasons between 1986 and 1995). The authors reported

that practice-related ACL injuries were more common

during the pre-season months of July and August, with

constant numbers for the remaining season. Game-related

ACL injuries were highest in August and December.

Accordingly, increased injuries were reported in hot com-

pared to cold conditions. Orchard and Powell [62] evalu-

ated 5,918 National Football League games (seasons

between 1989 and 1998) and found that weather conditions

had no influence on the risk of ACL sprains in natural

grass. However, the risk was lower in artificial turf of open

stadiums during cold weather (with no effect of whether

the playing surface was wet or dry) compared to hot

weather conditions. The incidence of ACL sprains was

lower during the cooler months of the season in open sta-

diums (both AstroTurf and natural grass) but not in domes

[62].

There are a number of studies published on the influence

of weather conditions in injuries in male athletes partici-

pating in Australian football [58–61]. Orchard et al. [59]

evaluated 2,280 matches (from 1992 to 1998) and found

that senior grade matches, high water evaporation in the

month before the match, and low rainfall in the year before

the match were associated with increased risk of non-

contact ACL injuries in male players. These results were

confirmed by the same group 2 years later [60]. Using the

penetrometer measuring system for ground hardness, early

season matches and matches played at a warmer weather

Pubmed 1821EMBASE 1361Cochrane 140TOTAL 3322

Title/Abstracts reviewed1827

Duplicates excluded1495

Full-text articles assessing risk factors

42

Excluded studies 1785Reasons for exclusion:-Animal studies-Cadaveric/in vitro studies-Epidemiological studies-Diagnostic studies-Therapeutic studies-Prevention programs-Review studies-Other type of studies-Involved only females-Compared females vs males

Studies included in this review article34

Added from reference lists10

Excluded 18Reasons for exclusion:-Not male vs male comparison-No ACL injuries involved

Fig. 1 Flow of information through the different phases of the

systematic literature search

Knee Surg Sports Traumatol Arthrosc

123

demonstrated a trend towards an increased risk of ACL

injury in Australian football [58]. Four years later, Orchard

et al. [61] published the results of the prospective follow-up

of their cohort of Australian football players. The authors

concluded that grass type more than ground hardness or

weather conditions would be responsible for the increased

risk of non-contact ACL injuries.

Finally, Walden et al. [80] compared the incidence of

ACL injuries in professional soccer players from Europe

depending on the climate. The authors separated northern

countries with a humid subtropical weather (hot humid

summer and cool winter) and marine west coast (warm

summer but no dry season), and southern countries with

Mediterranean climate (warm/hot and dry summer together

with wet winter). The authors found that the incidence of

ACL injuries was lower in northern compared to southern

countries.

Playing surface There was 1 study investigating playing

surface in Australian football, 9 in American football, and

Table 2 Assessment of the risk of bias in the included studies

Study A B C D E F G H I J K L M N Score

Bradley 02 N N U NA U U U N N NA N U NA NA 0

Brandon 06 N N U NA U Y Y Y N U N U NA U 2

Dragoo 12a N N U NA U U U N N U N U NA U 0

Dragoo 12b N N U NA U U U N N U N U NA U 0

Evans 12 N N U N U U Y Y N Y Y Y NA N 5

Everhart 10 N N Y N U Y Y Y N U N U NA U 3

Fuller 07a Y N U U U U N N N U N U Y U 2

Fuller 07b Y N U U U U N N N U N U Y U 2

Fuller 10 Y N U U U U N N N U N U Y U 2

Gomes 08 N N N NA U U Y Y N U Y U NA N 3

Hashemi 10 N N N NA Y Y Y N N U N U NA U 2

Hohmann 11 N N Y NA Y Y Y Y N U N U NA U 4

Hudek 11 N N Y NA U U Y Y N U N U NA U 2

Lambson 96 Y N Y U U U Y N N U N U Y U 4

Meyers 04 Y N U U N Y Y N N U N U Y U 4

Olsen 03 Y N Y U U U U N N U N U Y U 3

Orchard 99 N N U U U U U N N U N U NA U 0

Orchard 01a Y N U U U Y U U N U N U Y U 3

Orchard 01b N N U U U U U Y N U N U NA U 1

Orchard 03 N N U U U U U U N U N U NA U 0

Orchard 05 N N U U U U U Y N U N U NA U 1

Philippon 12 N N Y NA U Y Y U N U N U NA U 3

Powell 12 N N U U U U U U N U N U NA U 0

Scranton 97 N N U U U U U Y N Y N U NA U 2

Sheehan 12 N N U NA U N U Y N NA N U NA U 1

Souryal 93 Y N U NA U Y Y Y N U N Y Y N 6

Stijak 08 N N Y U U U Y N N NA N U NA U 2

Teitz 97 N N U NA Y Y U Y N U N U NA U 3

Todd 10 N N U NA U N Y Y N U NA Y NA U 3

Uhorchak 03 Y N U N Y Y Y Y N U Y Y Y N 8

Walden 13 Y N U N U U U Y N U U U Y N 3

Woodford-Rogers 94 N N Y NA U U U N N U N U NA U 1

Zazulak 07a Y N Y U Y Y Y N N U N U Y U 6

Zazulak 07b Y N Y U Y Y Y N N U N U Y U 6

A, Prospective; B, Concealed allocation; C, Similarities at baseline; D, Participant blinding; E, Data collector blinding; F, Outcome assessor

blinding; G, Previous knee injuries excluded; H, Results specified for non-contact injuries; I, No influence of other risk factors; J, Acceptable

compliance; K, Dropout reasons reported; L, Acceptable dropout rate; M, Duration of intervention comparable; N, Intention-to-treat analysis

Knee Surg Sports Traumatol Arthrosc

123

1 in rugby and handball, respectively. The approximate

number of subjects involved in the study of weather con-

ditions was 6,387 (although some studies did not specify

the total number of subjects involved).

Regarding Australian football, Orchard et al. [61]

observed that the type of grass was related to non-contact

ACL injuries in male athletes. The authors found that

Bermuda (couch) grass was associated with a higher risk of

this injury compared to Rye grass.

There are several studies investigating the risk of ACL

injuries in American football depending on the playing

surface [11, 22, 23, 31, 32, 51, 62, 66, 70]. Powell and

Table 3 Summary of risk factors for anterior cruciate ligament injuries in male athletes

Risk factors Main findings N Quality* Potential preventive strategy

Environmental

Weather conditions Higher ACL injuries in dry conditions 5 1 (0–3) Wetting of the playing surface

Lower ACL injuries in cold weather of open stadiums 1 0 (-) –

Northern European countries lower ACL incidence compared to

southern European countries

1 3 (-) N-M preventive training

Playing surface Bermuda grass higher risk of injury than Rye grass 1 1 (-) Wetting of the playing surface

or plant Rye grass

Artificial turf higher risk of injury than natural grass 3 0 (0–0) Use of natural grass fields

Natural grass higher risk of injury than artificial turf 1 2 (-) Use of artificial turf fields

No influence of natural and artificial surfaces on injury risk 5 2 (0–4) –

Footwear Edge-type foot designs higher risk of injury 1 4 (-) Use of flat, screw-in, or pivot

disk foot designs

Anatomical

Intercondylar notch

width

Narrow intercondylar notch higher risk of injury 2 7 (6–8) N-M preventive training

No influence of intercondylar notch width on injury risk 2 4 (3–5) –

Greater anterior outlet width and the presence of bone ridge in

intercondylar notch, greater risk of injury

1 3 (–) N-M preventive training

BMI Higher BMI increases the risk of injury 1 3 (–) Decrease weight before

engaging in sports

Posterior tibial slope Higher posterior medial tibial slope higher risk of injury 2 2 (2) N-M preventive training

No influence of posterior medial tibial slope on injury risk 4 2.5

(2–4)

–

Higher posterior lateral tibial slope higher risk of injury 3 2 (2) N-M preventive training

Femoral neck-head

offset

Higher alpha angle higher risk of injury 1 3 (-) N-M preventive training

Foot and ankle No influence of navicular drop and calcaneal eversion on injury risk 1 1 (-) –

N-M preventive training

Knee laxity Higher knee laxity higher risk of injury 1 1 (-) –

No influence of knee laxity on risk of injury 1 8 (-) N-M preventive training

Generalized joint

laxity

Generalized joint laxity higher risk of injury 1 8 (-)

Neuromuscular

Muscle strength and

recruitment

No influence of quadriceps and hamstring strength (either concentric

or eccentric) on injury risk

1 8 (-) –

Biomechanical

Hip range of motion Lower hip range of motion higher risk of injury 1 3 (-) N-M preventive training

Body positioning No influence of trunk displacement on injury risk 1 6 (-) –

Higher COM-BOS distance and limb angle (hip flexion) higher risk

of injury

1 1 (-) N-M preventive training

Lower trunk angle (more erected position) higher risk of injury 1 1 (-) N-M preventive training

Trunk

proprioception

No influence of trunk proprioception on injury risk 1 6 (-) –

ACL anterior cruciate ligament, N number of studies, N-M neuromuscular, BMI body mass index, COM-BOS centre of mass to base of support

* Median (range) of the quality score of studies involved in each risk factors

Knee Surg Sports Traumatol Arthrosc

123

Schootman found an increased risk of ACL injuries in

artificial turf compared to natural grass after evaluating

National Football League games between 1980 and 1989

[66]. In contrast, Scranton et al. [70] found an increased

risk of non-contact ACL injuries in natural grass compared

to artificial turf during games of the National Football

League. A more recent investigation reported no differ-

ences in rates of non-contact ACL injuries between natural

grass and artificial surface throughout games of a 9-year

period [11]. These results were supported in the study by

Orchard and Powell [62], who found no increased risk of

ACL sprains in natural grass compared to dome and open

turf. However, when considering practice sessions, an

increased proportion of ACL injuries was reported on

natural (82 %) compared to artificial (18 %) surface [11].

Meyers et al. [51] evaluated male football of 8 high schools

over 5 competitive seasons and found non-significant dif-

ferences for both isolated and non-isolated ACL injuries

between artificial turf and natural. Similarly, Fuller et al.

[32] found no differences between artificial turf and natural

grass for the incidence of ACL injuries in male players,

either in practice and games [31]. Dragoo et al. [22, 23]

reported a descriptive epidemiology study over a period of

5 years in National Collegiate Athletic Association football

male athletes. The rate of ACL injuries in artificial surfaces

was increased relative to natural grass for both pre-season

and in-season periods, but not in the post-season period

[22, 23]. The increased risk of ACL injury in artificial

surfaces compared to natural grass was present in games,

scrimmages, and practices, and across all divisions [23].

The risk was higher in games compared to practice, and in

scrimmages compared to regular practice and walk-

throughs [22]. Regarding the specific type of artificial turf,

third-generation surfaces (with fill) had more risk of ACL

injury compared to first- and second-generation surfaces

(without fill) [23].

Regarding the evidence for other sports, an investigation

was performed to compare the risk of ACL injury between

artificial turf and natural grass in male rugby players during

2 seasons of match injuries sustained by 6 teams competing

in Hong Kong’s Division 1 and training injuries sustained

by 2 teams in the English Premiership [30]. The authors

reported no significant differences in ACL injuries between

artificial surface and natural grass. The risk of ACL injuries

in male handball athletes of the 3 top divisions of Norway

during 7 seasons was evaluated relative to the playing

surface (wooden floors, generally with lower friction, and

artificial floors, generally with higher friction) [57]. The

authors found that the type of floor was not a risk factor for

ACL injury in male handball players.

Footwear There is only 1 study included in this section.

Lambson et al. [48] reported a prospective study aimed to

evaluate torsional resistance of football cleats and its

relationship to ACL injuries in high school American

football players. The study included a sample of 3,119

players during the 1989–1991 competitive seasons. The

foot cleat designs evaluated were: edge (longer irregular

cleats placed at the peripheral margin of the sole with a

number of smaller pointed cleats positioned interiorly), flat

(cleats on the forefoot are the same height, shape, and

diameter, such as found on the soccer-style shoe), screw-in

(seven screw-in cleats of 0.5 inch height and 0.5 inch

diameter), and pivot disk (a 10-cm circular edge is on the

sole of the forefoot, with one 0.5-inch cleat in the centre)

[48]. The authors reported that the edge designs produced

significantly higher torsional resistance than the other

designs. In addition, this foot cleat design was significantly

associated with a higher risk of ACL injuries compared to

the other designs.

Internal risk factors

Anatomical risk factors

The evidence found for anatomical risk factors for ACL

injuries in male athletes was mainly related to morphologic

characteristics and knee laxity. The former has to do with

both knee (intercondylar notch and posterior tibial slope

characteristics) and hip morphology. A total of 13 refer-

ences were found for anatomical risk factors: 5 for in-

tercondylar notch width, 1 for body mass index, 9 for tibial

slope, 2 for knee laxity, and 1 for each hip morphology,

foot and ankle morphology, and generalized joint laxity.

Morphologic characteristics The 14 studies included in

morphologic characteristics involved approximately 3,891

participants, distributed in the following activities: Amer-

ican football (1 study), military subjects (3 studies), com-

bination of sports (2 studies), and undetermined activities

(3 studies).

The principal morphologic characteristics related to

ACL injuries reported for males are intercondylar notch

and posterior tibial slope. Regarding intercondylar notch

morphology, Souryal et al. [73] compared, in a prospective

(level II evidence) cohort study, the notch width index

between injured and uninjured high school male athletes.

The authors reported that ACL-injured males had signifi-

cantly narrower intercondylar notch compared to uninjured

subjects. In contrast, a similar investigation involving a

sample of 40 males (mean age 39 years, range 19–70)

found that in subjects with non-contact ACL injuries, there

were no differences in notch width index between the

injured and uninjured side [76]. In 2003, Uhorchak et al.

[79] reported in a prospective study with young military

cadets that injured subjects had significantly lower notch

Knee Surg Sports Traumatol Arthrosc

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width, tibial eminence width, notch width index, and tibial

eminence width index compared to non-injured males

cadets. The authors found no differences in notch width/

eminence width ratio were reported [79]. Interestingly, the

authors found a 7.8-fold increased risk for non-contact

ACL injury in men with both narrow femoral notch and

generalized joint laxity compared to those without both

factors [79]. Evans et al. [26] reported a retrospective

comparative study with young, athletic subjects, and found

no differences between both injured and non-injured males

for height, condylar width, notch width, and notch width

index. Injured subjects had significantly higher weight and

body mass index compared to uninjured subjects. A narrow

notch width was associated with elevated body mass index

and was also a significant risk factor for males. Everhart

et al. [27] compared the notch morphology between sub-

jects with non-contact ACL injuries (cases) and a sample of

matched controls for age, sex, height, and weight. The

authors found that healthy men had greater anterior femoral

notch outlet width compared to injured men. The presence

of an anteromedial bone ridge in the femoral notch

increased the risk of non-contact ACL injury in men. In

addition, healthy men had a ridge thickness significantly

lower compared to injured men.

There are several retrospective investigations that have

evaluated the influence of posterior tibial slope on the risk

of non-contact ACL injuries in males. Brandon et al. [12]

reported a prognostic case–control study where they found

that males with ACL injuries demonstrated increased

posterior-inferior medial tibial slope when compared to

male controls. In contrast, another study reported ACL-

injured young males (not necessarily athletes) demon-

strated increased posterior tibial slope of the lateral plateau

(but not medial plateau) along with increased difference of

the tibial slope on the lateral and medial tibial plateaus

compared to non-injured control males [74]. In a similar

study, Hashemi et al. [39] compared the morphology of the

proximal tibia between ACL-injured (cases) and uninjured

(controls) males (not necessarily athletes). The authors

reported that injured males demonstrated increased medial

tibial slope, lateral tibial slope, and medial tibial depth

compared to uninjured males. In another case–control

study, Todd et al. [78] found no differences in posterior

tibial slope of the medial plateau between injured and

uninjured males. Similarly, Hudek et al. [46] found that

posterior lateral (but not medial) tibial slope in men with

non-contact ACL injuries was significantly higher relative

to control males. The authors also found that the meniscal

slope of injured subjects in medial and lateral plateaus was

significantly different compared to control subjects [46].

These authors introduced the measure of the meniscal slope

(marked by the superior margins of the anterior and pos-

terior horns in each meniscus). A comparison of the values

of the posterior tibial slope of the medial plateau between

males with and without ACL injuries was also reported by

Hohmann et al. [44]. As previously reported by other

authors [46, 74, 78], no significant differences in posterior

medial tibial slope between ACL-injured and control

groups were noted [44].

Recently, a retrospective investigation was performed to

determine the potential influence of the femoral neck-head

offset in the risk of ACL injuries [63]. The sample included

50 consecutive patients with primary ACL rupture and 50

consecutive patients with non-ACL injury (i.e. meniscus

tear, cartilage defect) [63]. The authors reported that male

patients with alpha-level angle over 60� were at a signifi-

cantly higher risk of ACL injury compared to male patients

with an alpha angle of 60� or less [63].

Finally, some aspects related to foot and ankle align-

ment were evaluated by Woodford-Rogers et al. [82]. The

authors compared the navicular drop (millimetre) and

calcaneal eversion in stance (degrees) of high school and

college male football players between the healthy limb of

non-contact ACL-injured and ACL-non-injured subjects.

They found no differences in navicular drop and calcaneal

eversion between injured and non-injured male football

players [82].

Knee laxity There were 2 studies reporting information

on the effects of knee laxity and generalized joint laxity on

the risk of ACL injuries. Both studies involved a total

sample of 767 individuals (28 American football players

and 739 military cadets).

Woodford-Rogers et al. [82] found that knee laxity (KT-

1000) of the uninvolved knee of subjects with an ACL

injury was increased relative to uninjured subjects. In

contrast, Uhorchak et al. [79] found no significant differ-

ences in specific knee laxity (KT-2000) between injured

and uninjured young male subjects. The authors also

reported that males with generalized joint laxity had a

relative risk of non-contact ACL injuries significantly

higher compared to subjects without generalized joint

laxity. They presented the relative risk of non-contact ACL

injuries in subjects with the combination of risk factors: the

risk of injury was significantly higher in subjects with

narrow notch width associated to generalized joint laxity

and specific knee laxity.

Neuromuscular risk factors

There was only 1 study included in this section. Uhorchak

et al. [79] reported a prospective investigation evaluating

several potential risk factors for non-contact ACL injuries

in a sample of 739 young male West Point cadets from the

United States Military Academy. Among other parameters,

the authors assessed the concentric and eccentric isokinetic

Knee Surg Sports Traumatol Arthrosc

123

strength of the knee extensors and flexors at 60 deg/sec

[79]. They observed no significant differences in quadri-

ceps and hamstrings strength, in either concentric or

eccentric muscle contractions, between non-contact ACL-

injured and non-injured males [79].

Biomechanical risk factors

The evidence found for biomechanical risk factors for ACL

injuries in male athletes was related to trunk and hip. Only 4

studies were found, which investigated the role of hip range

of motion (1 study), trunk and hip position (2 studies), and

trunk proprioception (1 study) on the risk of injury.

Trunk The investigation on the influence of trunk bio-

mechanics on the risk of ACL injuries in male athletes has

been reported in 3 studies. These studies involved a total

sample of 177 subjects from several types of sports (results

not specified by sport).

Zazulak et al. [85] reported a prospective biomechani-

cal-epidemiological prognosis study aimed to investigate

the influence of deficits in neuromuscular control of the

trunk to predict knee injury. Male athletes were tested for

trunk displacement after sudden force release and followed

for 3 years. Of the 137 male athletes initially evaluated, 14

male had knee injuries but there were only 2 ACL injuries.

The authors found no significant associations in maximal

trunk displacement (lateral, extension, and flexion trunk

displacement) between ACL-injured and uninjured males.

[85]. In a parallel publication, the same group found that

there were no significant differences in average error of

active proprioceptive repositioning of the trunk between

ACL-injured and ACL-uninjured male athletes [86].

In a recent publication, a case–control study was per-

formed that aimed to assess dynamic sagittal plane trunk

control during ACL injuries [71]. The authors conducted a

video analysis that compared movie captures of 20 athletes

performing a one-legged landing manoeuvre that resulted

in a torn ACL with matched movie captures of 20 athletes

performing a similar manoeuvre that did not result in an

ACL disruption (controls) [71]. They measured the dis-

tance of the centre of mass to base of support (normalized

by femur length) and measured the limb and trunk angles.

The trunk angle was defined as the angle from the vertical

to the centre line of the trunk [71]. The limb angle was

defined as the angle between the vertical and the thigh

(represented as the line from the centre of the knee joint to

the centre of the hip joint) [71]. A positive trunk and/or

limb angle indicated that the trunk and/or limb was rotated

anteriorly relative to the vertical [71]. ACL-injured males

had higher centre of mass to base of support distance and

limb angle, and lower trunk ankle, compared to uninjured

subjects. Essentially, injured subjects landed with a more

erect trunk and with more hip flexion when compared to

uninjured subjects.

Hip There was only 1 study included in this section. Gomes

et al. [34] investigated the association between hip range of

motion and non-contact ACL injuries in a case–control study

involving 100 male non-professional soccer players. Hip

range of motion was assessed in a supine position and with 908of both hip and knee flexion. Findings were analysed

according to 2 cut-off points (70� and 80� of total internal-

external rotation sum) [34]. After adjusting for age, the

authors found reduced hip range of motion in injured com-

pared to control athletes, especially for internal rotation. Also,

there were significantly more subjects with\70� and\80� of

total internal-external hip rotation sum in the non-contact

ACL-injured group compared to the non-injured group [34].

Discussion

The principal findings of this systematic review related to

the risk factors for ACL injury in male athletes are: (1)

most of the evidence is related to environmental and ana-

tomical risk factors; (2) dry weather conditions may

increase the risk of non-contact ACL injuries in male

athletes; (3) artificial turf may increase the risk of non-

contact ACL injuries in male athletes; (4) higher posterior

tibial slope of the lateral tibial plateau may increase the risk

of non-contact ACL injuries in male athletes; and (5) there

is a lack of evidence regarding neuromuscular and bio-

mechanical risk factors for male athletes. The investigation

of both factors in relation to the risk of ACL injuries is an

area wide open for exploration in the future years.

The number of studies available in the literature evalu-

ating risk factors for ACL injuries in male athletes is much

lower compared to what is published for female athletes [3,

17, 38, 42, 65, 68]. Moreover, several of the reviewed

studies were not focused to compare injured and uninjured

male athletes, but to compare females vs. males for some

specific parameter [12, 26, 27, 63, 76, 85, 86]. These

studies were included because specific comparisons

between injured and uninjured males were also provided,

but it can be concluded that the investigation of risk factors

for ACL injuries in the male athlete has not awakened

much interest in the scientific community. Although the

risk of ACL injury is higher in the female athlete [3, 17, 38,

42, 65, 68], the relevance of the investigation of these

factors in males is important given the high overall number

of male athlete participants [33]. There are many studies

investigating neuromuscular and biomechanical risk factors

in males, but the data are limited which include ACL injury

as an outcome. Therefore, these studies could not be

included in this review because clear conclusions on the

Knee Surg Sports Traumatol Arthrosc

123

influence of these factors on the risk of ACL injuries in

males cannot be drawn [2, 7–9, 13, 14, 16, 20, 21, 25, 28,

29, 47, 50, 64, 75, 81, 83, 84]. Prospective coupled bio-

mechanical-epidemiological data are even more spars in

male athletes. Including only those studies comparing

either prospectively or retrospectively injured and unin-

jured males, the evidence is much more limited, and has

been summarized in Table 3. As shown, most of the

included studies belong to environmental and anatomical

risk factors. Although neuromuscular and biomechanical

risk factors offer the greatest potential to support injury

prevention and likely provide a significant contribution to

risk, there is limited evidence available on these related

risk factors in males at this point. Interestingly, biome-

chanical risk factors have strong evidence for their rela-

tionships to ACL injury risk in female athletes [43, 85], and

these risk factors appear interrelated [41].

The investigation on the influence of environmental risk

factors for non-contact ACL injuries in male athletes may

have some limitations which include potential confounding

factors for weather conditions (type of surface itself, type of

shoe, biomechanical or neuromuscular risk factors), lack of

control for weather conditions where injuries did not occur,

differing type of activity (games or practice) played in

different type of surface (natural grass vs. artificial turf),

limited number of ACL injuries observed, or the inclusion

of both contact and non-contact ACL injuries [22, 23, 30,

62, 69, 70]. It is important to consider in the investigations

that included contact injuries in their analyses may have

limited relevance relative to measurement of injury risk

reduction from training. With contact injuries, the influence

of potential risk factors may be hidden because an external

load from other players likely has limited potential to be

effected by neuromuscular training aimed to prevent injury

[59]. It would be, therefore, mandatory to include this

information in any study dealing with risk factors or

preventive strategies for ACL injuries in male athletes.

However, only 14 of 33 studies reported the results for non-

contact ACL injuries (Table 2). Cumulatively, there does

not appear to be consensus regarding the effects of extrinsic

factors on ACL injury risk in male athletes. In addition, the

extent to which these factors can be modified is limited

which supports future investigations focused on risk

reduction based on intrinsic risk factors, specifically mod-

ifiable causative factors for ACL injury in male athletes.

Anatomical risk factors for ACL injury in male athletes

may have the limitation of potential influence of other non-

controlled associated risk factors. Cases and controls are

often not matched for height, weight, and type of activities

at risk of non-contact ACL injury [39, 44]. Another con-

cern is the type of control ‘‘data’’ employed for the com-

parison with injured subjects [82]. In general, how

anatomical factors modify knee kinetics and kinematics are

not well understood. Overall, the study of the influence of

anatomical factors on the risk of non-contact ACL injuries

in males clearly needs more research, especially in athletes.

However, the potential for their modification via injury

prevention strategies is limited. The main interest of their

identification in an athlete would be to know those indi-

viduals in whom prevention programmes take even more

relevance to avoid ACL injury. Therefore, in athletes with

anatomical risk factors, prevention programmes empha-

sizing modification of neuromuscular and biomechanical

risk factors would be even more relevant to reduce injury

risk in male athletes.

As evidenced above, there is only 1 study pertaining to

neuromuscular control (level II evidence) [79]. Therefore,

there is not enough evidence to elaborate strong conclu-

sions on the influence of the neuromuscular system on non-

contact ACL injuries in male athletes. With respect to the

biomechanical risk factors category, the included studies

may have some limitations: low number of ACL injuries

observed, a mismatch between the time of injury and time

when the mechanism was assessed, difficulties at identi-

fying anatomical landmarks in clothed participants, and the

potential of a selection bias as the inclusion criteria were

based on a qualitative analysis [71, 85, 86]. Although there

is more information available for biomechanical compared

to neuromuscular risk factors (Table 3), this information is

still based on a low number of studies, therefore strong

conclusions can neither be elaborated.

Further research is clearly needed for neuromuscular and

biomechanical risk factors for non-contact ACL injuries in

male athletes. Some areas to be developed in the near future

regarding risk factors for non-contact ACL injury in males

are the comparison of injured and uninjured subjects for:

joint angles and moments during different playing actions

(landing, sidestep cutting, stop jump, etc.), muscle activa-

tion and recruitment patterns, muscle fatigue differences,

and trunk neuromuscular control, among others. Currently,

there is no clear explanation or robust model that consis-

tently demonstrates how all of the risk factors interact

because a reductionistic model does not consider the inter-

relationships and synergic interaction between them.

There are some potential limitations to this systematic

review. First, some studies may have been missed from the

current literature search. However, the employment of 3

databases and the thorough review of obtained references,

including a full-text reading of the most important refer-

ences, and the careful double-check of cited studies mini-

mized the risk. Second, many studies with the absence of

significance in the principal comparisons had a low number

of cases (ACL-injured males), so the risk of type-II error is

not negligible. Third, non-prospective studies may have a

high risk of bias, especially for those modifiable factors.

Case–control studies retrospectively look at some

Knee Surg Sports Traumatol Arthrosc

123

characteristics after cases (ACL-injured males) have been

identified. If the studied factors may be modified with time,

this comparison may be biased by the fact that the injury

elicited some changes in the knee. This is not the case for

most of the anatomical risk factors, but would be clearly

the case for neuromuscular and biomechanical risk factors.

In the latter, it is very important that adequate prospective

cohort studies are designed to assure confident conclusions,

controlling or adjusting the analysis for as much other risk

factors as possible. These types of studies would provide

the best causal-effect relationship between the factor and

the risk of non-contact ACL injuries.

Conclusions

The results of this systematic review indicate that ACL

injury in male athletes likely has a multi-factorial aetiol-

ogy. A thorough knowledge of these factors is crucial to

provide adequate prevention strategies aimed to decrease

the risk of this serious injury in the male athlete. The fol-

lowing conclusions may be elaborated regarding risk fac-

tors for non-contact ACL injuries.

• Most of the existing evidence is related to environ-

mental and anatomical risk factors for non-contact ACL

injuries.

• Dry weather conditions may increase the risk of non-

contact ACL injuries in male athletes.

• Artificial turf may increase the risk of non-contact ACL

injuries in male athletes compared to natural grass.

• Most of the studies of anatomical risk factors are not

specific for athletes.

• Higher posterior tibial slope of the lateral tibial plateau

may increase the risk of non-contact ACL injuries in

male athletes.

• The existing evidence for neuromuscular and biome-

chanical risk factors is low.

• The investigation of risk factors in male athletes,

especially neuromuscular and biomechanical, provides

possibility for future research.

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